discourse/lib/topic_retriever.rb

class TopicRetriever

  def initialize(embed_url, opts=nil)
    @embed_url = embed_url
    @opts = opts || {}
  end

  def retrieve
    perform_retrieve unless (invalid_host? || retrieved_recently?)
  end

  private

    def invalid_host?
      SiteSetting.normalized_embeddable_host != URI(@embed_url).host
    rescue URI::InvalidURIError
      # An invalid URI is an invalid host
      true
    end

    def retrieved_recently?
      # We can disable the throttle for some users, such as staff
      return false if @opts[:no_throttle]

      # Throttle other users to once every 60 seconds
      retrieved_key = "retrieved:#{@embed_url}"
      if $redis.setnx(retrieved_key, "1")
        $redis.expire(retrieved_key, 60)
        return false
      end

      true
    end

    def perform_retrieve
      # It's possible another process or job found the embed already. So if that happened bail out.
      return if TopicEmbed.where(embed_url: @embed_url).exists?

      # First check RSS if that is enabled
      if SiteSetting.feed_polling_enabled?
        Jobs::PollFeed.new.execute({})
        return if TopicEmbed.where(embed_url: @embed_url).exists?
      end

      fetch_http
    end

    def fetch_http
      user = User.where(username_lower: SiteSetting.embed_by_username.downcase).first
      return if user.blank?

      TopicEmbed.import_remote(user, @embed_url)
    end

end